V. N. Tkach

Production of the Y3Al5O12 transparent nanostructured ceramics

The nanocrystalline Y3Al5O12 ceramics with different phase compositions, microstructures, and optical characteristics have been prepared by low-temperature consolidation of the Y3Al5O12 powder at pressures from 6 to 7.7 GPa in the temperature range from 250 to 550°C. The conditions have been defined for obtaining transparent nanostructured ceramics Y3Al5O12 having grains of size 20–40 nm and a transmission coefficient in the visible region of 40–45%. The criteria for the transparency of the nanostructured ceramics have been formulated and the ways of an improvement of its optical characteristics are discussed.

Studies of the oxidation stability, mechanical characteristics of materials based on max phases of the Ti-Al-(C, N) systems, and of the possibility of their use as tool bonds and materials for polishing

Thermogravimetry and differential thermal analysis have been used to study the resistance to the air oxidation of high-density samples of Ti3AlC2, Ti2AlC and Ti2Al(C1−xNx) solid solutions. It has been shown that the Ti3AlC2 samples are more stable than Ti2AlC and Ti2Al(C1−xNx) solid solutions and as the nitrogen content of the solid solution increases to x = 0.75, the oxidation resistance decreases. The following characteristics have been exhibited by the material containing 89 wt % Ti3AlC2 (the rest being Al2O3 and TiC) having density 99% of theoretical: bending strength Rbm = 500 MPa, compressive strength Rcm = 700 MPa, fracture toughness KIc = 10.2 MPa·m0.5, hardness HRA = 70 GPa, HV = 4.6 GPa, Young modulus = 149.4 ± 28.7 GPa. After sintering with diamonds or cBN (50 wt %) at 5.5–7.7 GPa and 1350–1960°C for 0.07–1.0 h the Ti3AlC2 MAX phase decomposes to form TiC and TiAl or TiB2 and a thin layer of Al4C3 forms at the interface with diamond. The Al4C3 decomposition in a composite material due to the interaction with the air moisture results in the crack initiation along the diamond perimeter, which brings about the material fracture in 1–2 weeks. It has been found that the Ti3AlC2 powder is efficient for polishing natural and synthetic jewelry crystals and competitive in polishing efficiency and quality with ACM 2/1 grade diamond.

Mechanical properties of HfB2 whiskers

Nanoindentation has been used to study mechanical behavior of HfB2 whiskers 10–20μm in diameter in a directionally reinforced ceramics. For comparison a bulk (0001) HfB2 single crystal 5 mm in diameter has been tested. For both the samples a pop-in due to the nucleation of dislocations in a previously dislocation-free region under the indent has been observed. It has been shown that for a HfB2 whisker in reinforced ceramics the critical load of the elastoplastic transition is twice as high as for a bulk HfB2 single crystal and, the maximum shear stress, at which the nucleation of the first dislocation loop in a HfB2 whisker occurs, approaches to the theoretical shear strength value. The observed effect has been caused by the higher structural perfection of whiskers as compared with a bulk sample. Hardness and elastic modulus of the HfB2 whisker are higher than that of the bulk crystal by 10 and 3%, respectively.

The diamond-tungsten carbide polycrystalline composite material

The sintering of a diamond composite from diamond and tungsten powders of submicron and nanosizes at high pressure and temperature has been discussed. It has been shown that as a result of the diamond and tungsten interaction tungsten carbide particles, which are chemically bonded to diamond particles, form in spaces between them. Over the whole volume of the composite the structure has been observed, where tungsten carbide and diamond grains are regularly placed and are uniform in size. An addition of a coarse diamond component to the composite has been found to contribute to an increase of the composite wear resistance. The optimal sintering parameters and formulation of this composite have been defined.

Distribution of local deformations in diamond crystals according to the analysis of Kikuchi lines profile intensities

The histogram method has been proposed to determine strains in diamond crystals by the analysis of reflection electron diffraction bands in Kikuchi patterns. The anisotropy has been defined of the local strain distribution on the surface of two diamond samples produced by the temperature gradient method in the Fe-Al-C system and by growing on a surface of a statically synthesized diamond single crystal (Ni-Mn-C) in the Mg-C + Bor system.

A novel composite abrasive material of unmarketable natural diamond powder

The results of the studies of physical properties and size characteristics of unmarketable natural diamond powders have been discussed. A method to compact such powders by physicochemical synthesis at pressures below ambient has been described. Morphometric and operating characteristics of powders produced from the resultant compacts by dispersion have been analyzed. It has been shown that these powders are suitable for using in diamond-abrasive tools and pastes.

The effect of diffusion processes on the interaction in multicomponent silicate systems

The results of studying diffusion processes in silicate multicomponent systems, which are widely used as binders in variously purposed composite materials, have been generalized and described. It has been shown that the effective diffusion coefficients of alkali and alkaline-earth cations in a diffusion pair are not a single-valued function of the ion radius of the cation but depend on structural factors and on the energy of the cation bonding with other ions of the melt.

A study of surface condition of sapphire (α-Al2O3) workpieces upon their finishing operation

An experimental investigation of surface condition parameters (surface roughness Rmax and depth of affected surface layer F) of sapphire workpieces upon their finishing operations is carried out using advanced measurement instrumentation; some results are provided. The factors have been found which have an effect on the above-mentioned parameters and relation between them in the course of machining; this makes it possible to calculate machining allowances for each operation and design the machining process such that the highest requirements to optical surfaces will be met.

The influence of physical-mechanical characteristics of AS6 synthetic diamond powders on wear resistance of grinding tools

The paper addresses physical-mechanical characteristics of AS6 diamond grits synthesized in the Ni-Mn-C and Fe-Si-C systems. These diamonds vary in magnetic properties, mechanical characteristics, amount of impurities and intracrystalline inclusions. The authors demonstrate a potential of using the results of investigation of morphometric characteristics of dimond powders for their efficient application in grinding tools. A relationship has been established between these characteristics and the tool wear resistance in diamond grinding.

Local deformation in diamond crystals defined by the Fourier transformations of Kikuchi patterns

A new approach has been proposed to determination of local strains in crystals using patterns of electrons backscattering diffraction (Kikuchi patterns) and discrete two-dimensional Fourier transformations. The new approach has been tested in studying diamond samples synthesized under different conditions. The results obtained have been found to agree well with the data obtained by other methods.